1. Field of the Invention
The present invention relates to the sphere of stratigraphic interpretation of seismic data. More particularly, the invention relates to processes for automatic detection of seismic reflectors showing pertinent characteristics for seismic stratigraphy interpretation, such as sedimentary layer convergence characteristics.
2. Description of the Prior Art
Seismic imagery is a method of observing the architecture of the subsoil, notably used in the fields of petroleum exploration and characterization of underground natural hydrocarbon reservoirs. This technique is based on the emission of acoustic signals in the subsoil and recording of the propagated signals and reflected on particular acoustic reflectors. These signals are processed so as to form a two or three dimensional seismic image. This seismic image is a series of vertical records referred to as “seismic traces”. These seismic traces represent the amplitude of the signal received as a function of time. The recorded signals generally correlate from one trace to the next, which is translated, in a seismic image, into sub-horizontal lines, more or less rectilinear, thick and continuous, referred to as lineations or reflectors. These lines represent the interfaces between sedimentary layers. A two-dimensional seismic image corresponds to a vertical section of the subsoil. An example of a two-dimensional seismic image is given by FIG. 2. The two dimensions are depth Z and a sub-horizontal geographic direction X.
From the observation of the seismic image, the seismic stratigraphy interpreter has a certain number of empirical rules for finding clues allowing reconstruction of the sedimentary history of the studied zone, and therefore to find subsoil zones likely to be oil traps. The interpreter notably uses the analysis of the “brightness” and of the length of the reflectors, of their position and their orientation in relation to neighboring reflectors (parallel reflectors can indicate a regular deposit at a great water depth and of a limited petroleum interest, fan-shaped convergent reflectors indicate a progressive tilt of the formation during deposition, having implications on the distribution of the sediments), and on the way the latter interrupt on one another. Analysis of the geometry of reflectors is for example the subject of a basic seismic stratigraphy article:    Vail. P. R., R. M. Mitchum, and S. Thompson, III, 1977, [<<] “Seismic Stratigraphy and Global Changes of Sea Level, part 4: Seismic Stratigraphy and Global Changes of Sea Level, in Payton”, C. E. (ed.), Seismic Stratigraphy—Applications to Hydrocarbon Exploration: Amer. Assoc. of Petrol. Geologists, Memoir 26, p. 83-97.
The interpretation stage is often very delicate and it entirely depends on the interpreter's sedimentologic expertise. There are few tools allowing assistance in this task, which with a more complex seismic image often shows several hundred reflectors in 2D and several thousands in 3D.
Several projects have been undertaken in the past years, providing seismic image analysis methods for facilitating the stratigraphic interpretation procedure. The following document that summarizes these methods can be mentioned for example:    S. Copra and K. J. Marfurt, [<<] Seismic Attribute Mapping of Structure and Stratigraphy”, Distinguished Instructor Series, no 9, EAGE, 2006.
The work of the “TriTex” project can also be mentioned:    European Research Project Funded by the European Commission, Directorate-General Information Society, “Automated 3D Texture Content Management in Large-Scale Data Sets”, Project No.: IST-1999-20500, Super-final report.
This work provides image analysis methods that convert an initial image into a final image. They therefore are only a preprocessing stage before interpretation. In fact, the seismic interpreter must eventually provide a “pick” of the seismic image, that is manually extract therefrom, by digitizing, objects referred to as seismic reflectors, expressed in vectorized form: a reflector is vectorized by a 2D broken line, or by a 3D discretized surface. The methods presented above provide a transformation of the initial image that facilitates picking, but they do not perform this picking.
Among the image analysis methods provided, some are particularly interesting for seismic stratigraphy because they relate to the detection of convergences in seismic images. Examples thereof are:    T. Randen et al, [<<] “New Seismic Attributes for Automated Stratigraphic Facies Boundary Detection”[>>], SEG-98, New Orleans, La., Expanded Abstracts, September 1998,    A. Barnes, [<<] “Attributes for Automating Seismic Facies Analysis”, SEG Technical Program, Expanded Abstracts—2000—pp. 553-556.
The feature these methods have in common is the preliminary determination of a “field of directional vectors” of the image to be analyzed. This is, at each pixel, in determining if the image has a texture oriented in a square neighborhood centered on the considered pixel, and in calculating the direction vector(s) thereof. The latter calculation determines a single mean local orientation (Randen, Barnes).
In Barnes' work, quantification of the reflectors convergence is obtained by calculating the divergence of the field of directional vectors. In Randen's work, the directional field is subjected to a search for flowlines whose density expresses the divergence or the convergence of the reflectors.
These methods work with mean values calculated in sliding neighborhoods of fixed and predetermined shape and size, which is not perfectly suited for seismic image interpretation. In fact, seismic images have the following particular feature: some reflectors, rectilinear or not, divide the image into two domains characterized by very different image textures wherein the dominant orientations on either side of the reflector are different. This is due to the fact that seismic reflectors correspond to sedimentary interfaces representing sudden historical changes in the sedimentologic conditions. For example, a reflector indicates the transition between a sandy environment of eolian dunes and a more clayey marine environment, possibly separated in the course of time by an erosion phase.
The aforementioned methods do not allow separation of the analysis performed in a neighborhood placed according to the position of a reflector (completely above or completely below a reflector for example), because they do not involve the notion of “reflector”. They provide information that “averages” the data obtained from a neighborhood that includes different environments and that cannot be characterized by the same parameters.
The invention is a method for stratigraphic interpretation of seismic data allowing characterization of the data by parameters specific to each seismic reflector present in the seismic data. The invention notably allows automatic separate reflectors so as to extract pertinent characteristics for stratigraphic interpretation, in order to detect very precisely particular sedimentary layer layouts, such as onlap or toplap type configurations (convergence of sedimentary layers linked with the sedimentary deposit type).